1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus and method for inspecting the dimensional characteristics of fuel assembly grids.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the opposite ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved inner straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid. The outer straps conventionally have springs integrally formed into the metal thereof which project into respective ones of the cells disposed along the perimeter of the grid.
The manufacture of a fuel assembly grid is an intricate operation, requiring not only the assembling of the straps in interleaved fashion to form the grid but also their retention together in precise positions relative to one another during subsequent welding thereof. The newly-manufactured grid must meet high standards in terms of its cell size (that is, the distance between opposite springs and dimples within each cell), envelope, squareness and dimple perpendicularity for it to be able to properly perform its function in the fuel assembly. Thus, the grid must be meticulously inspected to ensure that such standards are met. Heretofore, grid inspection has required the performance of a series of time-consuming, essentially manual, procedures.
Grid inspection, as practiced heretofore, has constituted an impedient to improvement of overall productivity of fuel assembly manufacture. Consequently, a need has emerged to improve and automate the way in which fuel assembly grid inspection is performed.